Control system for air heater



Feb. 5, 1963 A. H. RoBsoN CONTROL SYSTEM FOR AIHHEATER Filed Oct. 30, 1959 United States Patent O 3,076,604 CONTROL SYSTEM FOR AIR HEATER Aubrey H. Robson, Rock Island, Ill., assignor to American Air Filter Company, Inc., Louisville, Ky., a corporation of Delaware Filed Oct. 30, 1959, Ser. No. 849,882. 6 Claims. (Cl. 236-10) This invention relates to a fuel and combustion control system for portable air heaters particularly suitable for heating relatively small confined spaces such as vans, trailers and other shelters used by the military and others in various climates.

Such air heaters may be employed in areas where the outside air temperature is at least as low as -65 F., as well as in other areas having only moderately cold temperatures. Thus, it is desirable that such heaters not only have ample heating capacity for rapidly warming the space initially, but also be capable of maintaining the space at a comfort level after warm-up without causing wide variations in the space temperature.

One type of conventional air heater which has not been completely satisfactory for heating such .a confined space has been arranged to be operated in the conventional on-otf manner in accordance with the sensed air temperature in the space. This type of heater operates at 'maximum heating capacity and is alternately fired and shut off in response to a sensed air temperature respec- -tively below and above a comfort level in the space. One primary deficiency of such a heater is that the maximum .heating capacity has to be fairly low to prevent the discharge of unduly hot air into the space after the comfort .level has been reached. This of course results in a longer warm-up period when the space is cold.

`However even after the space has been warmed to a lcomfort level, if the heater operates at maximum heating capacity with even a relatively high air volume the air discharged into the space will be at an excessively high temperature. For example, a heater having a 50,000 B.,t.u. per hour heating capacity and a Ventilating air volurne of 500 cfm. (cubic feet per minute) will initially 'provide heated air of about only 5 F. when the inlet air is 65 F. However this same heater will provide out- .let `air at about 160 F. when the inlet air is 70 F. Thus the normal heater adapted to provide on-of operation .not only requires a relatively long warm-up period, but .also results in the discharge of air at excessively high temperatures into the space after warm-up. Additionally, -with the on-otl' type of operation, the heat exchanger is subjected to the maximum tiring rate each v, time heat is required. To say the least, the alternately ori-oft discharge of such high temperature air into the occupied space does not promote a comfort condition.

Furthen the hot discharge air gives a sensation of being `rscorched and has a drying eiect on the eyes and nose. The present invention is directed to a heater of the :general type disclosed in HubbardUS. Patent 2,758,591

'Visl also provided with means to reduce the burner fire Vwhile still controlling lit in a modulating fashion to pro- "vide va lowered discharge air temperature after the space .has reached a selected comfort level temperature from sustained operation of the .heater at the selected discharge air temperature. Specilically, the invention contemplates the provision of parallel throttle valves in the 3,076,604 Patented Feb. 5,119.53

return line of a bypass type nozzle. One valve 'is arranged to attempt to control burner fuel discharge in accordance with variations in discharge air temperature from `a selected temperature in the discharge air temperature range, while the other valve attempts to control burner fuel discharge in accordance with variations in inlet air temperatures from another selected temperature in a relatively lower temperature range.

Since the heater of this invention is controlled in a modulating fashion as distinguished from an on-off fashion, the heater may have a relatively high maximum heating capacity and a relatively low air flow volume to produce rapid warm-up, and still be capable of delivering heated air at a reasonably low temperature after the desire space temperature is reached. For example, the invention may be applied to a heater having a heatiing capacity range of say, 20,000 to 100,000 B.t.u. per hour, and an air flow rate of 350 c.f.m. Thus, with the space temperature initially being -65 F., the discharge air at maximum heating capacity would be about 133 F.; while after the space has been warmed suticiently tcprovide heater inlet air at 70 lF., the minimum heating capacity would produce heater discharge air at about 123 F, Additionally the system is arranged so that if it is de sired to utilize the heater for supplying heated air at `a selected discharge temperature to a sp-acefrom which air is not returned to the heater, the fuel control valvfe means normally responsive to the heater inlet air temperature will automatically remain closed while the outlet temperature valve will control.

It is also noted that the heater control vsystem may include a novel arrangement of safety controls. l

The invention will be explained in connection with the accompanying drawing which is a diagrammatic view of a fuel system and heater embodying the present invention and which illustrates a preferred embodiment by Way of example.

Referring to the drawing, and in particular to that portion within the dotted linerectangle, a forced air blower 2 provides Ventilating air to be heated andwhich liows as indicated by the solid arrows, while blower 4 supplies a separate supply of combustion air flowing as-indicated by the dotted arrows. A burner and combustion chamber 6 in which combustion takes place receives the combusltion air and liquid fuel from jet nozzle 8, the burner. also -containing ignition electrodes 10 adjacent the jet orifice -by numeral 14, and the heated Ventilating air passes therethrough to a heated air discharge outlet 16 through 'which heated Ventilating air vis discharged into suitable .conveying ducts for delivery `to the rservedspace.-

One temperature vsensing ele-ment 18 disposed adjacent the heated air outlet 16 serves to control a discharge air temperature responsive throttle valve, while another temperature sensing element 20 is adapted to cause burner shut `down if a discharge air overheat occurs..

Since it is contemplated that the heater will be applied normally in circumstances wherein the heated air discharged into the schematically shown conditioned space 21 will be returned or recirculated to the heater, a suitable air conveying duct means 23 is connected to convey the air from the heater to the conditioned space, and the return air from the conditioned space to air inlet 22 of the heater. An element 24 provided in this inlet 22 senses inlet air temperature and controlsan inlet air temperature responsive throttle valve.

vthe valve closure member in a closed position.

-Passing now to the fuel system, a tank 26 containing a supply of gasoline or similar liquid fuel is connected by conduit or line 28 having fuel ilter 30 therein to fuel pump 32, which draws fuel from the tank and forces it into line 34. Line 34 is connected to a balanced regulating valve 36 which is adjusted to open at a predetermined pressure (e.g., 150 p.s.i.) to permit how of fuel at that pressure into nozzle supply line 3S leading to nozzle S.

A pair of parallel primary return or bypass lines 49 and 42 are connected to line 34 intermediate the pump and regulating valve. Each primary bypass line contains a two-position (open-closed) valve operable, when open, to bypass -fuel `from line 34 back to the fuel tank by way of tank return line 44. rIhus, when either valve is open, no fuel is supplied to nozzle supply line 38 since the pump 32 is unable to establish suicient fuel pressure in line 34 to open, or maintain in an open position, the pressure regulating valve 36. Both valve 46, herein characterized as the main burner fuel valve, and valve 48, herein characterized as the flameout-ignition failure valve, are closed manually when it is desired to initiate burner lire. When both valve 46 and 48 are closed, the pressure vin line 34 builds up suiciently to open regulating valve 36 and permit flow of fuel to line 38 supplying the nozzle. This same fuel pressure maintains valve 46 in a closed position when the operator releases the valve closing plunger 49. The valve 46 also operates as an overheat cut-out valve and opens in response to an excessive discharge air temperature sensed by element 20 and transmitted to valve 46 through capillary line 50'. VDetails of the structure of this burner fuel valve 46 and vits operational characteristics are disclosed in detail in the mentioned Hubbard patent.

The lameout-ignition failure valve 4S is of the general type disclosed in Marvin US. Patent 2,718,918 issued September 27, 1955. After burner lire has been initiated, the valve 4S is maintained in a closed position in response Vto a continued sensing of burner lire by thermocouple element 11 disposed in the lburner chamber 6. This element 11 functions as a thermoelectric generator and in response to said continued sensing of heat above a predetermined degree, causes electrical current to ilow through conducting line 52 to energize a solenoid within the valve body. The energized solenoid in turn latches So long as'suliicient heat is present to cause the generation of enough current to maintain the solenoid in an energized condition, the valve 48 will remain closed after the manu- `ally closing means is released. Thus, if burner re terminates after having once been established, the thermo- `couple will cool and consequently de-energize the solenoid so that valve 48 opens. Further, if burner re fails to be established when starting, release of the manually closing means of the valve 48 will result in the valve returning to an open position due to failure of the thermocoupleto generate the-required solenoid energizing current.

It'will be apparent then that since either valve 46 or :.48. is effective when open to'cause yfuel to bypass back to .the tank and thereby prevent or terminate fuel ow to nozzle Sthrough nozzle supply line 38, that an overheat,

yan ignition failure, or a flameout will cut off fuel ow to the burner.. Before passing, it is also noted that it -follows from theforegoing that if either valve 46 or valve 48 opens, the other valve will also open.

.A secondary bypass line 54 connects the regulating v'valve 36 to the tank return line 44. This line 54 serves,

when primary bypass valves 46 and 48 are closed and regulating valve 36 is open, to return to the tank the excess fuel above that required to maintain the pressure in the nozzle supply line 38.

Burner nozzle Sis of the bypass type which permits the rate of fuel discharged as a jet into the burner to be controlled by throttling fuel ow on the downstream side of the nozzle. Such a burner nozzle having a built-in bypass is illustrated and described in detail in the noted Hubbard patent.

With fuel supplied by line 38 at a constant pressure to the supply side of the nozzle, part of the fuel will be discharged through an orifice into the burner and part of it will pass through the built-in bypass to the return side of the nozzle. The rate of discharge into the burner will depend upon the fuel back pressure at the return side of the nozzle-a higher back pressure causing a higher rate of discharge into the burner.

The return side of the nozzle is connected to nozzle return or bypass line 56 provided with a check valve 58. The check valve prevents residual fuel, or an unbalanced pressure between the lines 56 and 38, from causing feedback of fuel to the nozzle after the heater has been shut down.

Single nozzle return line 56 is divided into a pair of branch or parallel nozzle return lines 60 and 62 having throttle valves 64 and 66 respectively therein. These throttle valves control the fuel discharge from the nozzle when fuel is being supplied through nozzle supply line 38. The valve 64 attempts to control fuel discharge into the burner at a rate which will produce a selected discharge air temperature at the outlet of the heater, and valve 66 attempts to control fuel discharge into the burner at a, rate which will produce a selected inlet air temperature at the inlet of the heater.

Whether the throttle valves 64 and 66 cooperate to increase the back pressure and thereby increase the rate of fuel discharge in the burner, or whether they oppose each other with one valve attempting to decrease fuel back-pressure while the other attempts to increase fuel back-pressure, will depend upon the sensed temperature of the air entering and leaving the heater. In other words, if the air temperature sensed by outlet element 18 associated with valve 64 is below the selected discharge air temperature, and if the air temperature sensed by the inlet element 24 associated with valve 66 is also below the selected inlet air temperature, both valves will be closed in an effort to maintain a high back pressure in nozzle return line A56 and thereby maintain a high rate of fuel discharge into the burner. Conversely, if both temperatures sensed by elements 18 and 24 are above the respectively selected temperatures, both valves will be opened in an attempt to decrease fuel backpressure in nozzle return line 56 and consequently decrease the rate of fuel discharge into the burner. However,as will be apparent from the following, normally both valves will be closed initially to obtain the maximum rate of fuel discharge into the burner when the heater is started, and then, after one or the other of the elements 18 or 24 senses a temperature exceeding the respective selected temperature, the valves will in effect operate in opposition insofar as one valve will be attempting to increase the fuel discharge rate while the other will be attempting to decrease it. It is here noted that in attaininga stable operating condition the valve attempting to increase fuel discharge will be closed or move to a closed position, and it is this characteristic of the system which is effective to establish modulating control in the other valve.

The throttle valves 64 and 66 are both of the general type disclosed in U.S. Patent 2,505,933, issued May 2, 1950 to Clarence A. Aughey et al. and therefore will be described only brieliy. As mentioned, each is controlled in response to the air temperature sensed by their respective sensing elements 18 and 24 and which are connected to the valves proper by capillary tubes 68 and 70 respectively. Each valve is also provided with temperature selection means for adjusting the compression of an internal valve spring against a valve diaphragm subject, on its opposite side, to the vapor pressure in the capillary line. Rotatable knob 72 is operable to vary internal spring ten sion and thus permit the selection of a desired temperature for lvalve 64 while knob 74 serves this function for valve 66.

.While the valves 64 and 66 operate in a similar fashion in that each valve operates in response to a sensed air temperature and attempts to vary fuel back-pressure in the nozzle return line 56 in response to the difference between a sensed and selected temperature, the valves are different in certain respects. In this connection, the sensing element y18 and line 68 associated with valve 64are charged to respond and e exert a modulating control on the valve within a relatively high air temperature range of say, about 120 F. to 180 F. In contrast thereto, sensing element `24 and line 70 associated with valve 66 are charged to respond and exert a modulating control on this valve within a lower air temperature range of say, 60 F. to90 F. Further, under certain conditions it may be found desirable that valve 66 be provided with a valve closure member and cooperating valve port or seat designed to exert progressively less resistance to fiow` in a valve opening movement than the corresponding elements in valve 64 in a corresponding movement. Such an arrangement permits valve 66 to control the burner lire in a reduced range below that attainable when valve 64 `controls, and permits quicker control by valve 66. It is noted that the prime limitation on such an arrangement is dictated by considerations of stable llame supporting fuel flow at relatively low fuel back-pressures. Since it is desirable that the inlet air temperature to the heater be maintained within narrower limits than the heater outlet air temperature, it is preferable that valve 66 respond more readily than valve 64 to small departures in temperature from the control point. This may be accomplished, in part at least, by utilizing a larger control diaphragm in valve 66 than in valve 64 so that a given degree of sensed temperature variation within limits permits a greater valve movement in valve 66 than valve 64.

, Before passing it is noted that while the inlet air sensing element 24 is -responsive to the temperature of the air returned to lthe heater inlet rather than to the temperature ofthe air in the conditioned space itself, it will be appreciated that with a closed air circuit the heater inlet air Vtemperature closely reects the space temperature.

y TheY fuel pump 32 may be driven by either an electric motor or through a gear box take-olf from an internal combustion engine. As shown in the drawing, an accessory drive electric motor 76 drives both the pump 32, and an ignition magneto 78 which is connected by line 80 to fuel ignition electrodes 10. If an internal combustion engine is used as` the prime mover for the heater, the ventilation and combustion air blowers may be combined in an integral assembly as illustrated in the mentioned Hubbard patent, and a make up air inlet opening 82 downstream from the sensing element 24 can be provided. If such an internal combustion engine is used, a back pressure valve 84 is provided in the tank return line 44, and an engine fuel line 86 is connected upstream therefrom.

When it is desired to start beater operation, ventilation air ow and combustion air ow are initiated. The accessory drive 76 operates pump 32 and magneto 78. With both llameout valve 48 and burner fuel valve 46 open, fuel drawn from tank 26 will bypass back to the tank by way of bypass lines 40 and 42 and tank return line 44.

When it is desiredto initiate combustion in the burner, both valves 46 rand 48 are manually closed. Thus, the fuel pressure in line 34 builds up to a level which causes the regulating valve 36 to open and supply fuel at the predetermined pressure to nozzle supply line 38. It will be noted that the burner fuel valve 46 need be manually held in a closed position only until the fuel pressure in 4the lines 34 and 40 builds up sufziently to hold the valve closed from fuel pressure. However, the flameout valve 48 must be maintained in a manually closed position until the burner fire has been established a sufficiently long time thatthermocouple element 11 generates power to hold valve 48 closed. This normally takes about 6 to 10 seconds. It is to be understood that so long as accessory drive 76 is operating ignition magneto 78, a spark will be provided at the ignition electrodes 10 to ignite fuel issuing from the nozzle.

For purposes of explaining the operation of the system, it will be assumed that the heater is connected to discharge the heated air from outlet 16 into a confined space such as a trailer van which is in turn connected to return the air from the van to the heater inlet 22. It will also be assumed that the discharge air temperature valve 64 is set to begin opening at 150 F. and the return air temperature valve 66 is set to begin opening at 70 F. Assuming that it is relatively cold and the van has been cold soaked at a temperature of, for example, below 0 F., when the heater is started both valves 64 and 66 will be closed. The closed valves will cause maximum back pressure in nozzle return line 56, and a correspondingly maximum rate of fuel discharge into the burner from nozzle 8. Assuming now that the selected discharge air temperature is exceeded before the heater inlet air reaches a temperature above the selected heater inlet temperature, the valve 64 will move to an open position. Since the conditioned space is still below lthe selected heater inlet temperature, the inlet air temperature sensing element 24 maintains valve 66 in a closed position while the valve 64 will be modulated back and forth in response to the sensed discharge air temperature in an attempt to maintain the discharge air temperature at the selected point. Then when the continued discharge of heated air results in the heater inletair temperature reaching and rising above 70 F., the sensing element Z4 associated lwith the return air temperature valve 66 will cause the valve 66 to open and attempt to 'decrease the back-pressure and correspondingly reduce the rate of discharge of fuel into the burner. This causes the discharge air temperature to be reduced and valve 64 will close in an effort to bring the discharge air back up' to its selected temperature. If closing of valve 64 causes the heaterinlet air to rise further above its selected temperature, valve 66 opens still further and insures that valve 64 will remain closed; Thus, in effect, valve 64 retreats from 'the system (i.e., moves to an entirely'closed position) in an attempt to bring the discharge air temperature up while valve 66 controls fuel flow to provide a heater inlet air tempera? ture at its set point. f.

From the foregoing it will be appreciated that a heate controlled as disclosed is capable of providing a relattively high discharge air temperature to give rapid warm- Yup of the conditioned space at a controlled discharge air temperature, and then, after the conditioned spaceha's yreached the desired temperature (as indicated by the reflected heater inlet air temperature) the burner lire is reduced to a value which provides just enough heat to "maintain the conditioned space `at the desired comfort Ilevel.

It is conceivable that with -a high heat loss from the space and the discharge'air temperature selector set at a valuel providing insufficient heat output to bring the conditioned space up to the desired temperature, that the valve 66would fail to assume control.` In this case, the individuals in the conditioned space would, after a time, be aware of thefact that the conditioned space wastoo cold and the discharge air temperature selector should be adjusted upwardly. v i

If duringheater operation the burner lire fails,V the thermocouple 11 will cool and de-energize the solenoid in valve 48. Valve 48 would thus open and fuel would lbypass therethrough back to the tank. If the discharge air temperature rises above the predetermined overheat setting, the burner fuel valve 46 opens so that fuel bypasses back to the tank. Under these conditions, as with a arneout, the fuel ow to main nozzle supply line 38 is terminated. Manual shut down of the burner may be effected by pulling out plunger 49 of valve 46.

If it is desired to heat a space from which return air is not delivered to the heater inlet, the valve 66 responsive to inlet air temperature may be set to a value exceeding the ambient air temperature so that it will remain in a closed position and modulation of burner fire will be controlled exclusively by valve 64.

Having described my invention I claim:

1. In an air heating system including a substantially closed air circulation path: an air heater in said path; a fluid fuel burner for said air heater including7 a bypass type nozzle; means for supplying fuel to said nozzle; nozzle return conduit means including parallel fuel return conduits connected to receive fuel bypassed through said nozzle; a first throttle valve in one of said parallel return conduits; means for controlling said first throttle valve by opening and closing said valve in response to discharge air temperatures produced by said burner respectively above and below a discharge air temperature selected from a first temperature range; a second throttle valve in the other of said parallel return conduits; and means for controlling said second throttle valve by opening and closing said valve in response to the temperature of air returned to said heater from said space being respectively above and below an inlet air temperature selected from a second relatively lower temperature range corresponding to a normal comfort temperature range; said first and second valve control means'including means operating, `under conditions of said burner providing a discharge air temperature corresponding substantially with said selected discharge air temperature and an inlet air temperature below said selected inlet air temperature, to control fuel back-pressure and corresponding burner fire by modulating said first throttle valve and maintaining said second throttle valve in closed position, and in response to said selected discharge air temperature producing an inlet air temperature above said selected air inlet temperature, to transfer control of said fuel backpressure and corresponding burner fire by modulating said second throttle valve and closing said first throttle valve.

2. In an air heating system for a confined space including a substantially closed air circulation path: an air heater in said path; a liuid fuel burner for said air heater including a bypass type nozzle; means for supplying fuel to said nozzle; nozzle return conduit means connected to receive fuel bypassed through said nozzle; a first throttle valve in said nozzle return conduit means; a second throttle valve in said nozzle return conduit means in parallel with said first throttle valve; temperature responsive means to control said first and -second throttle valves including, means to control-said first valve in response to departures of the heater outlet air temperature from a selected outlet air temperature, and means to control said second valve in response to departures in temperature of the heater inlet air received from Asaid confined space from a selected heater inlet air temperature, said first andsecond valvevcontrol means including means operating, in response to a condition of both said outlet and inlet air tmeperatures being below said selected temperatures, to hold said valves closed, and under conditions of said outlet temperature exceeding said selected outlet temperature with said inlet temperature remaining below said selected inlet air temperature to open said first valve and hold said second valve closed to that variations of fuel flow into said burner are controlled by said first valve, and in response to said inlet air temperature exceeding said selected inlet air temperature to open said second valve and close said first valve so that variations of fuel flow into said burner are then controlled by said second valve.

3. The system specified in claim 2 wherein: said second valve control means includes means for varying said fuel flow at an accelerated rate relative to said first valve Vcontrol means in response to a given departure in air temperature from a selected air temperature.

4. In an air heating system including a substantially closed Vair circulation path: an air heater in said path; a fluid Vfuel burner for said air heater including a bypass type nozzle; means for supplying fuel to said nozzle; nozzle return conduit means including parallel fuel return conduits connected to receive fuel bypassed through said nozzle; a first throttle valve in one of said parallel return conduits; a second throttle valve in the other of said parallel return conduits; temperature responsive lmeans to control said first and vsecond throttle valve in response to outlet and inlet air temperatures of said air heater and operable when a `selected heater outlet air temperature is attained prior to attainment of a selected heater inlet air temperature to open said `first throttle valve to reduce fuel discharge into said burner so as to obtain said selected heater outlet air temperature while maintaining said second throttle valve closed, and operable in response to the subsequent rise of said heater inlet air temperature above said selected heater inlet air temperature, to close and maintain closed said first throttle valve and to open said second throttle valve to yfurther vreduce said fuel discharge into said burner so as to maintain said selected heater inlet air temperature irrespective of the demands of said first throttle valve to increase fuel 4discharge into said burner.

-5. In an air heating system including a substantially closed air circulation path; an air heater in said path; a fluid fuel burner for said air heater including a bypass type nozzle; means for supplying fuel to said nozzle; nozzle return conduit means connected to the return side of said nozzle to receive and return to said fuel source fuel bypassed through said nozzle; throttle valve means for controlling fuel discharge from said nozzle into said burner, said throttle valve means including a first and second throttle valve in parallel in said nozzle return conduit means; temperature responsive means to control said first and second throttle valve in response to departures of heater outlet and heater inlet temperatures from selected outlet and inlet temperatures respectively, said temperature responsive means controlling said fuel discharge into said burner in response -to the outlet temperature produced by said burner so long as said heater inlet air temperature is below said selected inlet air temperature, and in response to said inlet air `temperature so'long as said heater inlet air temperature is not below said selected inlet air temperature; primary bypass conduit lmeans connected to permit return of fuel to said fuel source and prevent fuel flow to said nozzle supply conduit, said bypass conduit means including first and second parallel valve means adapted to be closed in a normal operation of said -heater and operable independently to open positions in response to discharge air temperature overheat and burner fire failure respectively.

6. vThe system of claim -5 wherein: said first parallel valve is a manually'closable, openly biased valve held in a closed position by fuel pressure n said bypass conduit means above a predetermined minimum; and said second parallel valve is a manually closable, openly biased valve held in a closed position in response to heat above a predetermined value produced in said burner.

References Cited in the file of this patent UNITED STATES PATENTS 1,633,066 Breese June 2l, 1927 2,262,825 Welliver Nov. 18, 1941 2,758,591 Hubbard Aug. 14,1956 

1. IN AN AIR HEATING SYSTEM INCLUDING A SUBSTANTIALLY CLOSED AIR CIRCULATION PATH: AN AIR HEATER IN SAID PATH; A FLUID FUEL BURNER FOR SAID AIR HEATER INCLUDING A BYPASS TYPE NOZZLE; MEANS FOR SUPPLYING FUEL TO SAID NOZZLE; NOZZLE RETURN CONDUIT MEANS INCLUDING PARALLEL FUEL RETURN CONDUITS CONNECTED TO RECEIVE FUEL BYPASSED THROUGH SAID NOZZLE; A FIRST THROTTLE VALVE IN ONE OF SAID PARALLEL RETURN CONDUITS; MEANS FOR CONTROLLING SAID FIRST THROTTLE VALVE BY OPENING AND CLOSING SAID VALVE IN RESPONSE TO DISCHARGE AIR TEMPERATURES PRODUCED BY SAID BURNER RESPECTIVELY ABOVE AND BELOW A DISCHARGE AIR TEMPERATURE SELECTED FROM A FIRST TEMPERATURE RANGE; A SECOND THROTTLE VALVE IN THE OTHER OF SAID PARALLEL RETURN CONDUITS; AND MEANS FOR CONTROLLING SAID SECOND THROTTLE VALVE BY OPENING AND CLOSING SAID VALVE IN RESPONSE TO THE TEMPERATURE OF AIR RETURNED TO SAID HEATER FROM SAID SPACE BEING RESPECTIVELY ABOVE AND BELOW AN INLET AIR TEMPERATURE SELECTED FROM A SECOND RELATIVELY LOWER TEMPERATURE RANGE 